Oil well pipes such as tubing and casing used in the drilling of oil wells for crude oil and gas oil are typically connected to each other by threaded joints. In the past, the depth of oil wells was typically 2000 to 3000 meters, but more recently, the depth of oil wells may reach 8,000 to 10,000 meters in offshore oil fields and other deep wells.
A threaded joint for oil well pipes is subjected to various forces in its environment of use, including loads such as axial tensile forces caused by the weight of the oil well pipe and the threaded joint, pressures combined by internal and external pressures, and geothermal heat. Therefore, a threaded joint needs to be able to maintain gas tightness without breakage in such severe environments.
During the process of lowering tubing or casing into a well, a joint which is once fastened for connection often needs to be loosened or unfastened and then re-fastened. API (American Petroleum Institute) requires that there be no occurrence of galling, which is unrecoverable seizing, and that gas-tightness be maintained even if fastening (make-up) and unfastening (break-out) are repeated ten times for a tubing joint and three times for a casing joint.
One type of threaded joint for oil well pipes having excellent sealing properties is a threaded joint with a pin-box structure having a metal to metal contact portion forming a seal. In this threaded joint, typically, a pin is formed on the outer surface of each end of an oil well pipe. The pin has an externally threaded portion and a metallic contact portion without threads. The latter portion is hereunder referred to as an “unthreaded metal contact portion” and it is located on the front end of the threaded portion at the tip of the pipe. A box is formed on the inner surface of a separate, sleeve-shaped coupling member, and it has an internally threaded portion and an unthreaded metal contact portion on the rear of the threaded portion which respectively contact or mate with the corresponding threaded and unthreaded portions of the pin. When the pin is screwed into the box for fastening, the unthreaded metal contact portions of the pin and the box are made to tightly contact each other to form a metal seal.
As a matter of principle, a pin may be formed on one end of an oil well pipe and a box may be formed on the opposite end of the oil well pipe so that two oil well pipes can be connected to each other in series without using a separate coupling member. It is also possible to form an internal thread on the inner surface at both ends of an oil well pipe to define boxes, and to form an external thread on each end of a coupling member to define pins.
In order to guarantee adequate sealing properties by the metal seal of the above-described threaded joint in the environment of an oil well pipe, it is necessary to apply an extremely high surface pressure to the unthreaded metal contact portions during fastening. This high pressure makes it easy for galling to take place. Therefore, prior to fastening, a lubricating grease called a compound grease is applied to the metal contact portions and the threads in order to impart an increased resistance to galling and form a seal having improved gas tightness.
However, compound grease contains a large amount of a powder of a heavy metal such as Pb, Zn, or Cu, and when the applied grease is washed off, there is the possibility of its causing environmental pollution. In addition, the application of compound grease worsens the work environment and decreases the operating efficiency. Accordingly, a threaded joint which does not need the application of a lubricating grease such as compound grease is desired.
With respect to a threaded joint which does not use a lubricating grease, Japanese Published Unexamined Patent Applications Nos. 8-233164 (1996) and 9-72467 (1997) disclose a pin-box type threaded joint having a solid lubricant-resin coating formed on the metal contact portion of the box or pin. This coating is based on a solid lubricant and comprises a large amount of a solid lubricant such as molybdenum disulfide dispersed in a resin such as an epoxy resin. Japanese Published Unexamined Patent Application No. 11-132370 (1999) discloses a threaded joint having such a solid lubricant-resin coating in which the taper ratio of the threads is optimized.
However, it is not easy to form a solid lubricant-resin coating to a uniform thickness along the shape of the thread and the unthreaded metal contact portions of a threaded joint. If the coating is not formed with a prescribed uniform thickness, in those areas where the coating is too thick, the surface pressure exerted thereon during fastening of the threaded joint becomes too large, resulting in an increase in the torque required for fastening (make-up torque), or this may lead to deformation of the shape of the threads, and it becomes easy for galling to occur. On the other hand, in areas where the coating is too thin, poor lubrication and rusting tend to occur easily.
In an oil well, connection of oil well pipes to each other is frequently carried out in a state in which an oil well pipe to be connected is stood upright with the pin at one end of the pipe facing into the ground. At the time of fastening, it is not unusual for misalignment to occur in such a manner that the longitudinal axis of the pipe, i.e., the rotational axis during screwing of the pin, horizontally deviates from alignment with the axis of the box in which the pin is inserted, or slopes to the side from the upright position. In addition, when an oil well pipe is stood upright, rust or scale flakes peeled off from the internal surface or blasting particles introduced to remove rust or scale may fall down through the lumen of the pipe and adhere or deposit on the joint. Thus, fastening may take place in the presence of rust or scale flakes or blasting particles adhered on the threaded portion or the unthreaded metal contact portion.
Under conditions involving the above-described problems of misalignment and/or deposited foreign matters which occur frequently during fastening of pipes in an oil well, even if the solid lubricant-resin coating can be formed to a prescribed constant thickness, it is easy for galling to take place. This is because a solid lubricant-resin coating has poor extensibility and fluidity, and it may easily peel off. Under the above-described conditions, an excessive pressure may be applied locally on some areas in the threaded portion and unthreaded metal contact portion, thereby causing the solid lubricant-resin coating to peel off in the excessively pressurized areas. Thus, bare metal surfaces may be exposed in some areas of the joint, leading to the occurrence of galling.
On the other hand, when a greasy or oily lubricant which has inherently good fluidity is applied to a threaded joint, at the time of fastening of the joint, pressure acts on the lubricant confined in the gaps between threads or in valley portions of the surface to cause the pressurized lubricant to seep into the surrounding areas, and as a result the metal contact portions can be successfully lubricated. This effect cannot be expected with a solid lubricant-resin coating which inherently has poor extensibility and fluidity.
A lubricating grease is normally washed of and re-applied prior to each fastening operation. In contrast, a solid lubricant-resin coating is formed prior to the initial fastening operation and should be maintained up to the last fastening operation, so it has a problem with respect to rust prevention as described below.
FIG. 1 is a schematic diagram showing an oil well pipe 1 having a pin with an externally threaded portion 3 at both of its ends and a threaded coupling member 2 having a box with an internally threaded portion 4 at both of its ends.
The oil well pipe 1 is normally shipped in the state shown in FIG. 1, i.e., in a state in which the coupling member 2 is connected in advance to one end of the oil well pipe 1. Accordingly, the above-described solid lubricant-resin coating must be formed prior to connection of the threaded coupling member 2 to one end of the oil well pipe 1, which is carried out before shipping. The resulting oil well pipe/coupling member assembly has a pin or box at each end which is not connected to a mating box or pin, and such a pin or box is often exposed to rain water during transport or during storage. The solid lubricant-resin coating does not have sufficient rust preventing properties to completely protect the pins and boxes in such situation and rusting can easily occur. If a compound grease is applied prior to shipment partly for the purpose of rust prevention, rusting can be prevented more effectively.
Due to the poor lubricity of rust, and due to swelling or peeling of a solid lubricant-resin coating accompanying the formation of rust, the make-up torque required to fasten a rusted joint becomes even more unstable, thereby causing galling to take place easily and gas tightness to decrease.